H2C═CH—COOCH2CH2N(CH3)2  (I)is obtained by the transesterification reaction between a lower alkyl acrylate of formula (II): CH2═CH—COOR1 in which R1 represents the methyl or ethyl radical, and N,N-dimethyl aminoethanol (DMAE), according to the following reaction scheme:CH2═CH—COOR1+HO—CH2CH2N(CH3)2DMAEA+R1OH  (1)
The reaction is generally carried out in the presence of an excess of lower alkyl acrylate and the reaction is displaced towards formation of DMAEA by distilling the lower alcohol R1OH in the form of a lower acrylate/R1OH alcohol azeotrope, which can advantageously be recycled, if its quality permits, to the lower acrylate production unit, said lower acrylate being produced by direct esterification of acrylic acid with the alcohol R1OH.
According to document EP 906 902, the azeotropic mixture resulting from distillation is purified on an ion-exchange resin, thus reducing its content of nitrogen associated with the presence of aminated by-products that may form during the transesterification reaction.
During synthesis of DMAEA by the transesterification reaction, there is generally formation of acetaldehyde CH3CHO as by-product. The acetaldehyde can then react with the alcohol R1OH to give an acetal, dialkoxyethane, according to the reaction:CH3CHO+2R1OHCH3CH(OR1)2+H2O  (2)
The dialkoxyethane is dimethoxyethane or diethoxyethane depending on whether the DMAEA is prepared from methyl acrylate, or from ethyl acrylate.
The acetaldehyde and dialkoxyethane produced during the synthesis of DMAEA are predominantly in the lower acrylate/R1OH alcohol azeotropic fraction, thus contributing to contamination thereof.
During recycling of the azeotropic fraction to the production unit for methyl acrylate or ethyl acrylate, the acetaldehyde, on coming into contact with the methanol or ethanol and the acid catalyst that are present for effecting the esterification reaction, is transformed to dialkoxyethane, which is added to the dialkoxyethane already present in the recycled azeotropic fraction. The result is production of methyl acrylate or ethyl acrylate heavily contaminated with dialkoxyethane. This applies in particular to ethyl acrylate, since diethoxyethane has a boiling point of the same order as that of ethyl acrylate.
There is therefore a need to find a method for eliminating some or all of the acetaldehyde and of the dialkoxyethane present in the azeotropic fraction generated during the synthesis of N,N-dimethyl aminoethyl acrylate from a lower acrylate, said fraction being recyclable to the lower acrylate production unit, otherwise the lower acrylate will be heavily contaminated with dialkoxyethane. Purification of the lower acrylate proves difficult, as the difference between the boiling point of the lower acrylate and that of the dialkoxyethane is relatively small.
Accordingly, it is difficult to produce an alkyl acrylate free from dialkoxyethane, if acrylate/alcohol fractions containing it or which contain its precursor acetaldehyde, are recycled to the alkyl acrylate production unit.
Elimination of acetaldehyde from the lower acrylate/alcohol fractions by simple reduction using a reducing agent for example of the type NaBH4, LiAlH4, sodium sulfite, is not conceivable in the present case. In fact, in the presence of lower acrylate, use of these reducing agents would lead to the formation of large amounts of Michael adducts (methyl methoxypropionate or ethyl ethoxypropionate). This would be reflected in a loss of alkyl acrylate and of lower alcohol (methanol or ethanol).